A typical vehicle seat belt system includes a length of belt webbing which is extensible about a vehicle occupant in a vehicle seat to restrain the occupant. Intermediate its ends, the belt webbing passes through a webbing guide, sometimes referred to as a D-ring, which is part of a webbing guide assembly mounted in the vehicle adjacent the vehicle seat. In the event of a collision, particularly a side impact collision, the head of the occupant can forcibly strike the webbing guide assembly.
The United States Department of Transportation Federal Motor Vehicle Safety Standard No. 201 requires such a webbing guide assembly to have a Head Impact Criterion (referred to as HIC(d)) value of under 1000 determined in accordance with Standard No. 201 (1996).